Interferons (IFN) are a class of cytokines that are best known for their antiviral and immunomodulatory activities, but they also function in cellular differentiation, proliferation and more recently as hormones implicated in pregnancy during the periimplantation period in ruminant species. Since primate placentas have been shown to produce interferons and also contain receptors for such compounds, administration of such IFNs during fertile reproductive cycles could alter or disrupt early placental function or fetal development. We have completed the second series of studies investigating the effects of two forms of recombinant human IFN (type 1a) on reproductive cycles, pregnancy and fetal development in rhesus macaques. Both forms of the IFN were equivalent in their effects on various reproductive parameters, including anovulation and alterations in serum hormone profiles during the specific stages studied.